Cavitation at a rotating marine propeller produces undesirable effects; the two principal ones being noise that is transmitted through the water and erosion of the blade material. In many ships it has been the conventional practice to pump air down the drive shaft for the propeller and to discharge it from holes around the blade at a controlled pressure and flow rate. The air has traditionally been fed from an inboard compressor through a slip ring and down the whole length of the shaft, but this arrangement suffers from a number of disadvantages. The air feed arrangements require a multiplicity of inter-section joints, all of which have to be sealed. Hydraulic systems may also pass down the shaft, and a loss of integrity of the air supply can give rise to problem with these systems. It has therefore been desired to deliver the air at a slip ring located outside the ship's hull, possibly in a space between the propeller and the aftmost shaft bearing, with the air delivered through a bearing support bracket such as a so called A-bracket by a route well separated from other vital systems, thereby resulting in a more economic installation having fewer serviceability problems.
A solution to this problem described in No. GB-A-2050278 is to locate a split slip-ring about the shaft with two annular sealing rings in the aft face thereof wiping against the forward face of the propeller assembly. The sealing rings are made of a composition comprising a phenolic resin and asbestos and a passage in the propeller hub leading to the propeller blades rotates in the annulus between the sealing rings, thereby giving a seal that is air-tight and maintains its integrity upon radial and axial movement of the propeller. But the arrangement described is relatively complex.